In general, this invention relates to camera exposure control systems. More particularly, it relates to a control system wherein there is provided digitally controlled timing circuitry including a decimal count storing memory.
A variety of different types of camera exposure control systems are known in the art. One type which has previously been proposed involves a digital approach to timing the closing of the camera shutter. According to this previously proposed approach, a number of data pulses are produced. The number of data pulses so produced is a function of the light intensity of an object to be photographed. The number of data pulses may also be a function of other photographic factors such as lens aperture setting and film sensitivity. Further according to this previously proposed approach, the data pulses are counted and stored in a digital memory.
Generally, the digital memories that have been proposed heretofore for this purpose provide for accumulating a binary count. Representative of such prior art digital memories are binary counter circuits and binary reversible (up/down) counter circuits. It has also been proposed to use a ring counter for the digital memory, with a separate memory element being provided for every stored data pulse.
Further according to this previously proposed approach, timing pulses commence to be generated upon the opening of the shutter. Then, when the number of timing pulses equals (or bears some other predetermined relationship to) the number of data pulses stored in the digital memory, a command signal is produced to effect closure of the shutter. Any one of a variety of comparison techniques may be used for comparing the timing pulses with the data pulses to detect this predetermined relationship. In any case, a longer exposure time is defined when more timing pulses occur before such detection, and a shorter exposure time is defined when fewer timing pulses occur before such detection. For a more detailed explanation of the foregoing and a disclosure of another of my inventions, reference is made to pending application U.S. Ser. No. 595,805, filed July 14, 1975, entitled CAMERA EXPOSURE CONTROL SYSTEM ADAPTED TO ADJUST BOTH THE DIAPHRAGM AND THE SHUTTER, which is assigned to the assignee of this invention.
It is desirable to control exposure time very precisely, and also to provide a display indicating the exposure time. This is especially so in connection with color photography. In particular, controlling exposure time in increments of 1LV will normally suffice where black-and-white film is used. However, when color film is used, it is desirable to control exposure time in increments as fine as 0.2 to 0.3 LV.
To achieve this more precise control, it is important to increase the scale factor or ratio according to which the number of data pulses varies relative to light intensity. In an arrangement where more data pulses are produced, it is necessary to increase the capacity of the digital memory. Memory capacity can be increased by providing a larger number of memory elements. However, to do this with previously proposed digital memories involves problems such as the following.
In circumstances where a binary counter circuit (inclusive of a binary reversible counter circuit) is used, an undesirably large number of decoder elements must be used in order to display the exposure time precisely. The resulting arrangement must therefore be complex. In other circumstances where a ring counter circuit or the like is used, a separate memory element has been required for every counted data pulse. Consequently, a very large number of memory elements must be used for precise control.
The digital memories described above have also been proposed for use in connection with a diaphragm preference type of exposure control system. There, an alarm is indicated in various circumstances. One way of indicating the alarm involves light emission of a display member arranged conveniently as in a viewfinder. Another way involves blocking the shutter release. Such an alarm is given when light intensity of a object to be photographed is too high to avoid an over-exposure. This circumstance of unacceptably high light intensity arises when the existing aperture setting admits so much light that overexposure is unavoidable even when the shutter operates at its highest shutter speed for the particular camera. Such an alarm is also given when the light intensity is too low to avoid a blurring of the photograph. This circumstance of unacceptably low light intensity arises when the existing aperture setting admits so little light that a very slow shutter speed is required for exposure and the long exposure time increases the risk of blurring incident to movement of the hands. With such an alarm system, however, it has inconveniently been necessary for the photographer to adjust the diaphragm manually.
As to what constitutes an acceptable exposure time, the following factors should be noted. One factor is the maximum available shutter speed for the particular camera. Another factor is the human one of how still the photographer can hold the camera. Still another factor is the compositional one of whether a large or small depth of field is desired. In general, one limit of acceptable exposure time is determined by the highest shutter speed available, and the opposite limit is determined by the susceptibility to blurring. Typically, where exposure time is in excess of 1/30 sec., blurring results from hand movements. Even when a tripod is used, however, limits are imposed because of film characteristics, particularly when color film is used.